1. Field of the Invention
The present invention is in the field of fabrication of semiconductor devices. More specifically, the invention is in the field of fabrication of HBT semiconductor devices.
2. Related Art
In a silicon-germanium (xe2x80x9cSiGexe2x80x9d) heterojunction bipolar transistor (xe2x80x9cHBTxe2x80x9d), a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer. The SiGe HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. Cutoff frequencies in excess of 100 GHz, which are comparable to the more expensive gallium-arsenide based devices, have been achieved for the SiGe HBT.
The higher gain, speed and frequency response of the SiGe HBT are possible due to certain advantages of silicon-germanium, such as a narrower band gap and reduced resistivity. These advantages make silicon-germanium devices more competitive than silicon-only devices in areas of technology where high speed and high frequency response are required.
To satisfy an ever-increasing demand for higher performance SiGe HBTs, device manufacturers have attempted to increase the above advantages of silicon-geranium. For example, by increasing the concentration of germanium in SiGe cystalline structure of the base of a SiGe HBT, the band gap in the SiGe base is correspondingly lowered. As a result of lowering the band gap in the SiGe base, the performance of the SiGe HBT desirably increases. However, increasing the concentration of germanium in the SiGe base also increases the strain in the crystalline structure of the SiGe base.
By way of background, the strain in the crystalline structure of the SiGe base results from epitaxially growing silicon-germanium crystal on top of a silicon crystal. If the strain in the crystalline structure of the SiGe base exceeds a critical threshold, the SiGe base becomes metastable. The properties of metastable SiGe are discussed in a paper by D. C. Houghton, xe2x80x9cStrain Relaxation Kinetics in Si1xe2x88x92xGex/Si Heterostructures,xe2x80x9d Journal of Applied Physics, Volume 70, pp. 2136-2151, dated Aug. 15, 1991. Exposure of the metastable SiGe base to high temperature in a required final rapid thermal processing (xe2x80x9cRTPxe2x80x9d) step utilizing a soak anneal process can result in strain relief via plastic flow between the silicon and silicon-germanium crystals. Should such strain relief occur, the coherence of the epitaxial SiGe base with the silicon substrate is degraded, which results in a loss of advantages provided by the SiGe HBT discussed above.
In a conventional approach, device manufacturers ensure that strain relief does not occur in the epitaxial SiGe base by limiting the germanium concentration and base thickness of the epitaxial SiGe base to prevent the SiGe base from entering a metastable state. By preventing the SiGe base from entering the metastable state through limiting the germanium concentration and base thickness, this conventional approach provides a SiGe HBT having a stable base that will remain stable when integrated into a BiCMOS process for a final RTP soak anneal to complete dopant activation and form the base-emitter junction. However, by limiting the germanium concentration and base thickness, the above approach undesirably limits the performance of the SiGe HBT.
Thus, there is a need in the art for a high performance HBT having a metastable epitaxial base, where the metastable epitaxial base remains a strained crystalline structure after being subjected to high temperature in a final RTP step.
The present invention is directed to method for integrating a metastable base into a high-performance HBT and related structure. The present invention addresses and resolves the need in the art for a high performance HBT having a metastable epitaxial base, where the metastable epitaxial base remains a strained crystalline structure after being subjected to a high temperature in a final RTP step.
According to one exemplary embodiment, a heterojunction bipolar transistor is fabricated by forming a metastable epitaxial silicon-germanium base on a collector. The thickness of the metastable epitaxial silicon-germanium base, for example, may be greater than a critical thickness. The metastable epitaxial silicon-germanium base, for example, may have a concentration of germanium greater than 20.0 atomic percent of germanium. For example, the metastable epitaxial silicon-germanium base may have a concentration of germanium greater than 30.0 atomic percent of germanium or may have a concentration of germanium approximately equal to 40.0 atomic percent of germanium. The heterojunction bipolar transistor, for example, may be an NPN silicon-germanium heterojunction bipolar transistor.
According to this exemplary embodiment, the heterojunction bipolar transistor is further fabricated by fabricating an emitter over the metastable epitaxial silicon-germanium base. The emitter, for example, may be polycrystalline silicon. The width of the emitter, for example, may be approximately 0.2 microns or 0.9 microns. The heterojunction bipolar transistor is further fabricated by doping the emitter with a first dopant. The first dopant, for example, may be arsenic. The fabrication of the exemplary heterojunction bipolar transistor continues by heating the metastable epitaxial silicon-germanium base in a spike anneal process so as to maintain the metastable epitaxial silicon-germanium base as a strained crystalline structure after the spike anneal process and so as to diffuse the first dopant to form an emitter-base junction. In another embodiment, the present invention is a structure for a heterojunction bipolar transistor resulting from the implementation of the above fabrication method. Other features and advantages of the present invention will become more readily apparent to those of ordinary skill in the art after reviewing the following detailed description and accompanying drawings.